Sql Commands


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Sql Commands

  1. 1. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... Basics of the SELECT Statement www.thecodexpert.com In a relational database, data is stored in tables. An example table would relate Social Security Number, Name, and Address: EmployeeAddressTable SSN FirstName LastName Address City State 512687458 Joe Smith 83 First Street Howard Ohio 758420012 Mary Scott 842 Vine Ave. Losantiville Ohio 102254896 Sam Jones 33 Elm St. Paris New York 876512563 Sarah Ackerman 440 U.S. 110 Upton Michigan Query 1 let’s say you want to see the address of each employee. Use the SELECT statement, like so: Conditional Selection To further discuss the SELECT statement, let's look at a new example table (for hypothetical purposes only): EmployeeStatisticsTable EmployeeIDNo Salary Benefits Position 010 75000 15000 Manager 105 65000 15000 Manager 152 60000 15000 Manager 215 60000 12500 Manager 244 50000 12000 Staff 300 45000 10000 Staff 335 40000 10000 Staff 400 32000 7500 Entry-Level 441 28000 7500 Entry-Level Relational Operators There are six Relational Operators in SQL, and after introducing them, we'll see how they're used: = Equal < or != (see Not Equal manual) < Less Than > Greater Than <= Less Than or Equal To >= Greater Than or Equal To The WHERE clause is used to specify that only certain rows of the table are displayed, based on the criteria described in that WHERE clause. It is most easily understood by looking at a couple of examples. Query 2 EMPLOYEEIDNO's of those making at or over $50,000
  2. 2. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... More Complex Conditions: Compound Conditions / Logical Operators www.thecodexpert.com The AND operator joins two or more conditions, and displays a row only if that row's data satisfies ALL conditions listed (i.e. all conditions hold true). Query 3 To display all staff making over $40,000, use: The OR operator joins two or more conditions, but returns a row if ANY of the conditions listed hold true. Query 4 To see all those who make less than $40,000 or have less than $10,000 in benefits, listed together, use the following query: AND & OR can be combined, for example: Query 5 Find all managers whose salary is greater than 60000 and benefits are greater than 12000 IN & BETWEEN An easier method of using compound conditions uses IN or BETWEEN. Query 6 For example, if you wanted to list all managers and staff: Query 7 To list those making greater than or equal to $30,000, but less than or equal to $50,000, use: Query 8 To list everyone not in this range, try: Using LIKE Query 9 Look at the EmployeeStatisticsTable, and say you wanted to see all people whose last names started with "S"; try: Joins In this section, we will only discuss inner joins, and equijoins, as in general, they are the most useful. For more information, try the SQL links at the bottom of the page. Good database design suggests that each table lists data only about a single entity, and detailed information can be obtained in a relational database, by using additional tables, and by using a join.
  3. 3. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... First, take a look at these example tables: www.thecodexpert.com AntiqueOwners OwnerID OwnerLastName OwnerFirstName 01 Jones Bill 02 Smith Bob 15 Lawson Patricia 21 Akins Jane 50 Fowler Sam Orders OwnerID ItemDesired 02 Table 02 Desk 21 Chair 15 Mirror Antiques SellerID BuyerID Item 01 50 Bed 02 15 Table 15 02 Chair 21 50 Mirror 50 01 Desk 01 21 Cabinet 02 21 Coffee Table 15 50 Chair 01 15 Jewelry Box 02 21 Pottery 21 02 Bookcase 50 01 Plant Stand Keys First, let's discuss the concept of keys. A primary key is a column or set of columns that uniquely identifies the rest of the data in any given row. For example, in the AntiqueOwners table, the OwnerID column uniquely identifies that row. This means two things: no two rows can have the same OwnerID, and, even if two owners have the same first and last names, the OwnerID column ensures that the two owners will not be confused with each other, because the unique OwnerID column will be used throughout the database to track the owners, rather than the names. A foreign key is a column in a table where that column is a primary key of another table, which means that any data in a foreign key column must have corresponding data in the other table where that column is the primary key. In DBMS- speak, this correspondence is known as referential integrity. For example, in the Antiques table, both the BuyerID and SellerID are foreign keys to the primary key of the AntiqueOwners table (OwnerID; for purposes of argument, one has to be an Antique Owner before one can buy or sell any items), as, in both tables, the ID rows are used to identify the owners or buyers and sellers, and that the OwnerID is the primary key of the AntiqueOwners table. In other words, all
  4. 4. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... of this "ID" data is used to refer to the owners, www.thecodexpert.com buyers, or sellers of antiques, themselves, without having to use the actual names. Performing a Join The purpose of these keys is so that data can be related across tables, without having to repeat data in every table--this is the power of relational databases. For example, you can find the names of those who bought a chair without having to list the full name of the buyer in the Antiques table...you can get the name by relating those who bought a chair with the names in the AntiqueOwners table through the use of the OwnerID, which relates the data in the two tables. Query 10 To find the names of those who bought a chair, use the following query: DISTINCT and Eliminating Duplicates Let's say that you want to list the ID and names of only those people who have sold an antique. Obviously, you want a list where each seller is only listed once--you don't want to know how many antiques a person sold, just the fact that this person sold one (for counts, see the Aggregate Function section below). This means that you will need to tell SQL to eliminate duplicate sales rows, and just list each person only once. To do this, use the DISTINCT keyword. First, we will need an equijoin to the AntiqueOwners table to get the detail data of the person's LastName and FirstName. However, keep in mind that since the SellerID column in the Antiques table is a foreign key to the AntiqueOwners table, a seller will only be listed if there is a row in the AntiqueOwners table listing the ID and names. We also want to eliminate multiple occurrences of the SellerID in our listing, so we use DISTINCT on the column where the repeats may occur (however, it is generally not necessary to strictly put the Distinct in front of the column name). Miscellaneous SQL Statements Aggregate Functions I will discuss five important aggregate functions: SUM, AVG, MAX, MIN, and COUNT. They are called aggregate functions because they summarize the results of a query, rather than listing all of the rows.  SUM () gives the total of all the rows, satisfying any conditions, of the given column, where the given column is numeric.  AVG () gives the average of the given column.  MAX () gives the largest figure in the given column.  MIN () gives the smallest figure in the given column.  COUNT(*) gives the number of rows satisfying the conditions. Looking at the tables at the top of the document, let's look at three examples: SELECT SUM(SALARY), AVG(SALARY) FROM EMPLOYEESTATISTICSTABLE; This query shows the total of all salaries in the table, and the average salary of all of the entries in the table. SELECT MIN(BENEFITS) FROM EMPLOYEESTATISTICSTABLE WHERE POSITION = 'Manager'; This query gives the smallest figure of the Benefits column, of the employees who are Managers, which is 12500.
  5. 5. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... SELECT COUNT(*) www.thecodexpert.com FROM EMPLOYEESTATISTICSTABLE WHERE POSITION = 'Staff'; This query tells you how many employees have Staff status (3). Views In SQL, you might (check your DBA) have access to create views for yourself. What a view does is to allow you to assign the results of a query to a new, personal table, that you can use in other queries, where this new table is given the view name in your FROM clause. When you access a view, the query that is defined in your view creation statement is performed (generally), and the results of that query look just like another table in the query that you wrote invoking the view. For example, to create a view: CREATE VIEW ANTVIEW AS SELECT ITEMDESIRED FROM ORDERS; Now, write a query using this view as a table, where the table is just a listing of all Items Desired from the Orders table: SELECT SELLERID FROM ANTIQUES, ANTVIEW WHERE ITEMDESIRED = ITEM; This query shows all SellerID's from the Antiques table where the Item in that table happens to appear in the Antview view, which is just all of the Items Desired in the Orders table. The listing is generated by going through the Antique Items one-by-one until there's a match with the Antview view. Views can be used to restrict database access, as well as, in this case, simplify a complex query. Creating New Tables All tables within a database must be created at some point in time...let's see how we would create the Orders table: CREATE TABLE ORDERS (OWNERID INTEGER NOT NULL, ITEMDESIRED CHAR(40) NOT NULL); This statement gives the table name and tells the DBMS about each column in the table. Please note that this statement uses generic data types, and that the data types might be different, depending on what DBMS you are using. As usual, check local listings. Some common generic data types are:  Char(x) - A column of characters, where x is a number designating the maximum number of characters allowed (maximum length) in the column.  Integer - A column of whole numbers, positive or negative.  Decimal(x, y) - A column of decimal numbers, where x is the maximum length in digits of the decimal numbers in this column, and y is the maximum number of digits allowed after the decimal point. The maximum (4,2) number would be 99.99.  Date - A date column in a DBMS-specific format.  Logical - A column that can hold only two values: TRUE or FALSE. One other note, the NOT NULL means that the column must have a value in each row. If NULL was used, that column may be left empty in a given row. Altering Tables Let's add a column to the Antiques table to allow the entry of the price of a given Item (Parentheses optional):
  6. 6. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... ALTER TABLE ANTIQUES ADD (PRICE DECIMAL(8,2) NULL); www.thecodexpert.com The data for this new column can be updated or inserted as shown later. Adding Data To insert rows into a table, do the following: INSERT INTO ANTIQUES VALUES (21, 01, 'Ottoman', 200.00); This inserts the data into the table, as a new row, column-by-column, in the pre-defined order. Instead, let's change the order and leave Price blank: INSERT INTO ANTIQUES (BUYERID, SELLERID, ITEM) VALUES (01, 21, 'Ottoman'); Deleting Data Let's delete this new row back out of the database: DELETE FROM ANTIQUES WHERE ITEM = 'Ottoman'; But if there is another row that contains 'Ottoman', that row will be deleted also. Let's delete all rows (one, in this case) that contain the specific data we added before: DELETE FROM ANTIQUES WHERE ITEM = 'Ottoman' AND BUYERID = 01 AND SELLERID = 21; Updating Data Let's update a Price into a row that doesn't have a price listed yet: UPDATE ANTIQUES SET PRICE = 500.00 WHERE ITEM = 'Chair'; This sets all Chair's Prices to 500.00. As shown above, more WHERE conditionals, using AND, must be used to limit the updating to more specific rows. Also, additional columns may be set by separating equal statements with commas. GROUP BY & HAVING One special use of GROUP BY is to associate an aggregate function (especially COUNT; counting the number of rows in each group) with groups of rows. If aggregate functions are included in the SELECT clause <select list>, GROUP BY calculates a summary value for each group. First, assume that the Antiques table has the Price column, and each row has a value for that column. We want to see the price of the most expensive item bought by each owner. We have to tell SQL to group each owner's purchases, and tell us the maximum purchase price: SELECT BUYERID, MAX(PRICE) FROM ANTIQUES GROUP BY BUYERID;
  7. 7. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... Now, say we only want to see the maximum purchase price if the purchase is over $1000, so we use the HAVING www.thecodexpert.com clause: SELECT BUYERID, MAX(PRICE) FROM ANTIQUES GROUP BY BUYERID HAVING PRICE > 1000; . Example using the SUM function For example, you could also use the SUM function to return the name of the department and the total sales (in the associated department). The HAVING clause will filter the results so that only departments with sales greater than $1000 will be returned. SELECT department, SUM(sales) as "Total sales" FROM order_details GROUP BY department HAVING SUM(sales) > 1000; Example using the COUNT function For example, you could use the COUNT function to return the name of the department and the number of employees (in the associated department) that make over $25,000 / year. The HAVING clause will filter the results so that only departments with more than 10 employees will be returned. SELECT department, COUNT(*) as "Number of employees" FROM employees WHERE salary > 25000 GROUP BY department HAVING COUNT(*) > 10; Example using the MIN function For example, you could also use the MIN function to return the name of each department and the minimum salary in the department. The HAVING clause will return only those departments where the starting salary is $35,000. SELECT department, MIN(salary) as "Lowest salary" FROM employees GROUP BY department HAVING MIN(salary) = 35000; Example using the MAX function For example, you could also use the MAX function to return the name of each department and the maximum salary in the department. The HAVING clause will return only those departments whose maximum salary is less than $50,000. SELECT department, MAX(salary) as "Highest salary" FROM employees GROUP BY department HAVING MAX(salary) < 50000; More Subqueries
  8. 8. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... Another common usage of subqueries involveswww.thecodexpert.com a Where condition to include the Select the use of operators to allow output of a subquery. First, list the buyers who purchased an expensive item (the Price of the item is $100 greater than the average price of all items purchased): SELECT BUYERID FROM ANTIQUES WHERE PRICE > (SELECT AVG(PRICE) + 100 FROM ANTIQUES); The subquery calculates the average Price, plus $100, and using that figure, an OwnerID is printed for every item costing over that figure. One could use DISTINCT BUYERID, to eliminate duplicates. List the Last Names of those in the AntiqueOwners table, ONLY if they have bought an item: SELECT OWNERLASTNAME FROM ANTIQUEOWNERS WHERE OWNERID IN (SELECT DISTINCT BUYERID FROM ANTIQUES); The subquery returns a list of buyers, and the Last Name is printed for an Antique Owner if and only if the Owner's ID appears in the subquery list (sometimes called a candidate list). Note: on some DBMS's, equals can be used instead of IN, but for clarity's sake, since a set is returned from the subquery, IN is the better choice. For an Update example, we know that the gentleman who bought the bookcase has the wrong First Name in the database...it should be John: UPDATE ANTIQUEOWNERS SET OWNERFIRSTNAME = 'John' WHERE OWNERID = (SELECT BUYERID FROM ANTIQUES WHERE ITEM = 'Bookcase'); First, the subquery finds the BuyerID for the person(s) who bought the Bookcase, then the outer query updates his First Name. Remember this rule about subqueries: when you have a subquery as part of a WHERE condition, the Select clause in the subquery must have columns that match in number and type to those in the Where clause of the outer query. In other words, if you have "WHERE ColumnName = (SELECT...);", the Select must have only one column in it, to match the ColumnName in the outer Where clause, and they must match in type (both being integers, both being character strings, etc.). EXISTS & ALL EXISTS uses a subquery as a condition, where the condition is True if the subquery returns any rows, and False if the subquery does not return any rows; this is a nonintuitive feature with few unique uses. However, if a prospective customer wanted to see the list of Owners only if the shop dealt in Chairs, try: SELECT OWNERFIRSTNAME, OWNERLASTNAME FROM ANTIQUEOWNERS WHERE EXISTS (SELECT * FROM ANTIQUES WHERE ITEM = 'Chair');
  9. 9. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... If there are any Chairs in the Antiques column,www.thecodexpert.com a row or rows, making the EXISTS clause the subquery would return true, causing SQL to list the Antique Owners. If there had been no Chairs, no rows would have been returned by the outside query. ALL is another unusual feature, as ALL queries can usually be done with different, and possibly simpler methods; let's take a look at an example query: SELECT BUYERID, ITEM FROM ANTIQUES WHERE PRICE >= ALL (SELECT PRICE FROM ANTIQUES); This will return the largest priced item (or more than one item if there is a tie), and its buyer. The subquery returns a list of all Prices in the Antiques table, and the outer query goes through each row of the Antiques table, and if its Price is greater than or equal to every (or ALL) Prices in the list, it is listed, giving the highest priced Item. The reason "=" must be used is that the highest priced item will be equal to the highest price on the list, because this Item is in the Price list.
  10. 10. For useful Documents like this and Lots of more Educational and Technological Stuff Visit... For useful Documents like www.thecodexpert.com this and Lots of more Educational and Technological Stuff... Visit... www.thecodexpert.com